A sense resistor is used to measure current for a circuit and can be included in or with an integrated circuit (IC). Sense resistors are useful for providing information about one component of an IC to other components of the IC. Some components of an IC may be less “intelligent” components, such as speakers, that have no capability to make decisions or execute logical statements. Other components of the IC may be more “intelligent” components, such as controllers or processors, that have logic circuitry or general processing capability that can process decisions, generate control signals, and/or operate other components within the IC. A sense resistor is one monitoring tool that the more intelligent components may have access to when determining the condition or state of the less intelligent components. Design of the sense resistor is impacted by the IC's design, the IC's application, and/or the levels of currents that need to be measured or tolerated.
One conventional implementation of a sense resistor provides an external sense resistor coupled to components of an IC. FIG. 1 is a circuit schematic showing a circuit accessing an external sense resistor according to one example of the prior art. An example circuit 100 uses an external sense resistor 110 coupled to an IC 120. Circuit 100 includes an output driver 102 and a sense analog-to-digital converter (ADC) 104. Output driver 102 and sense ADC 104 may be part of, but not necessarily all of, circuit 100. Output driver 102 drives an output signal at its output through pin or ball 106. The external resistor 110 and a load 114 are coupled in series between pin or ball 106 and ground 122. An external capacitor 112 is coupled in parallel to the external resistor 110. The sense ADC 104 receives input through pins or balls 108 and 109, and the sense ADC 104 generates an output signal at output 111, which may be internal to an IC 120. The external resistor 110 is coupled across pins or balls 108 and 109. A short circuit switch 116 is coupled to pin 109, which may be used to test a robustness of the output driver 102 and resistor 110. Current through external resistor 110 generates a voltage across external resistor 110 that can be read as a voltage difference across the pins or balls 108 and 109 as an analog signal and converted to a digital signal by sense ADC 104. That digital signal may be used as feedback for an intelligent component, such as a controller in the IC 120.
In the design of circuit 100, external resistor 110 is not part of the IC 120, allowing the resistance value for external sense resistor 110 to be selected such that external resistor 100 can withstand or tolerate large short-circuit currents. However, a sense resistor with a high or higher resistance value typically equates to a larger sized resistor. Further, sizing a sense resistor with a high or higher heat dissipation value to allow tolerance of larger currents generally also results in a larger size resistor. A larger resistor consumes more space or area in an electronic device, which is disadvantageous and particularly disadvantageous in mobile devices. Also, as shown in example circuit 100, the use of external resistor 110 typically requires one or two additional pins or balls for the IC 120, shown as pins or balls 108 and 109. Additional pins or balls generally require additional layout and/or pin-out and also require additional space on the IC 120. Furthermore, external path 107 to the external resistor 110 and load 114 has an inherent inductance. If this inductance is not offset, then it may cause voltage ringing spikes for circuit 100. Thus, the use of external resistor 110 necessitates use of external capacitor 112 to offset the inductance. This external capacitor 112 consumes additional space and area in an electronic device that includes circuit 100.
An alternative solution to an external resistor is to integrate the resistor into the IC. FIG. 2 is a circuit schematic showing a circuit having an integrated sense resistor according to one example of the prior art. An example circuit 200 includes an IC 220 with an integrated resistor 202. Circuit 200 is similar to circuit 100 and includes output driver 102 and sense ADC 104. Similar to circuit 100, output driver 102 and sense ADC 104 are included in an IC, and output driver 102 drives an output signal to pin or ball 106. However, instead of external resistor 110 that is outside of the IC 120, circuit 200 includes integrated sense resistor 202 internal to the IC 220. The integrated resistor 202 is coupled between the output driver 102 and pin or ball 106. Internal nodes 204 and 206 couple integrated resistor 202 to sense ADC 104. Load 114 is coupled between pin or ball 106 and ground 122.
Designing integrated resistor 202 to withstand or tolerate large short-circuit currents requires the integrated resistor 202 to be large in size and consume space and area. The larger area for an IC makes it more challenging to symmetrically lay out the circuit to eliminate mismatches, such as mismatches due to having more than one source of current (e.g., current sources for a p-channel metal-oxide-semiconductor field-effect transistor (MOSFET) (PMOS) driver device and an n-channel metal-oxide-semiconductor field-effect transistor (MOSFET) (NMOS) driver device). The requirements for circuit symmetry further constrain the layout of circuit components, such as the layout for the integrated resistor 202, the current source, the pin or ball, and/or the internal nodes 204 and 206.
These symmetry requirements are illustrated in FIG. 3. FIG. 3 is a cross-sectional view showing an integrated circuit at a chip-scale package (CSP) level with an integrated sense resistor according to one example of the prior art. Integrated circuit layout 300 includes sense ADC 104 coupled to integrated sense resistor 202, which is coupled to pin or ball 106, such as a Chip-Scale-Package (CSP) output ball. The resistor value for integrated sense resistor 202 is a function of a ratio of its length L to its width W (e.g., L/W), and thus usually has a width W much greater than its length L. In order to maintain symmetry, the ratio L/W needs to be maintained as a constant. Thus, if the length L is made longer, then the width W also will need to be correspondingly made longer, and vice versa if they are made shorter. The integrated resistor 202 may be sensed by ADC 104 through internal sense points 306 and 308 located across the length L. Circuit layout 300 may also include NMOS driver 302 and PMOS driver 304 located at opposite sides of the sense ADC 104 and ball 106. Mismatches among NMOS driver 302 and PMOS driver 304 that provide multiple current sources can and generally do exist. Thus, a line of symmetry 310 needs to be maintained in order to maintain symmetry for integrated circuit layout 300 and reduce/eliminate mismatches. Also, if symmetry for integrated circuit layout 300 is not maintained, then the current values being sensed at the sense points 306 and 308 may not be accurate and/or may be inconsistent.
Shortcomings mentioned here are only representative and are included simply to highlight that a need exists for improved electrical components, particularly for sense resistors and integrated circuits (ICs) that utilize those sense resistors employed in consumer-level devices, such as mobile phones. Embodiments described herein address certain shortcomings but not necessarily each and every one described here or known in the art.